The present invention relates to a vapor deposition apparatus and method for applying a coating to a base material, and more particularly to a vapor deposition apparatus and method wherein small amounts of refractory elements are incorporated into a vapor deposition coating.
The thermal evaporation and condensation of solid materials such as metals to form a coating on a base material, commonly referred to as vapor deposition, is a relatively developed art. There are many sophisticated prior art techniques and apparatuses which permit such materials to be evaporated from a source and condensed to form a coating or layer on a substrate disposed a distance from the source. Such processes all involve heating a material to be evaporated to a temperature at which it has a significant vapor pressure, thus creating a vapor stream. Heating techniques include direct methods, such as heating the material to be deposited using resistance, induction, electron beam or laser beam means to melt all or some portion of the material to be evaporated, or indirectly, such as by heating the surface of a higher melting material and flashing the material to be evaporated off the hot surface. The evaporated material thereafter becomes condensed on the surface of the base material, thereby providing a coating thereon.
Refractory materials are often desired to be incorporated into coatings applied to the surface of components exposed to high temperatures, such as gas turbine components used inter alia in aircraft engines, to act as a protective coating in a process known as thermal barrier coating (TBC). Methods for depositing ceramic barrier coatings like zirconia (i.e. zirconium oxide) which serve as thermal barrier coatings are known in the art. For example, U.S. Pat. No. 5,773,078 to Skelly, commonly assigned to the assignee of the present invention, namely General Electric Company, discloses an improved method for depositing zirconium oxide from a zirconium oxide source onto a base material by means of physical vapor deposition, comprising the step of adding zirconium metal to a zirconium oxide ingot as the ingot is heated. Despite the improvements realized by the method of U.S. Pat. No. 5,773,078, use of a rare earth metal oxide such as zirconium oxide as an evaporant source causes problems due to release of the oxygen present in the oxide and difficulties in regulating the uniformity of composition in the resulting deposited condensate.
Where the starting material to be evaporated and applied as a coating to a base material is a multi-constituent alloy, other problems are encountered. In particular, the composition of the coated material when applied by the vapor deposition method as described frequently and undesirably was substantially different than the composition of the starting material, and/or the condensate would not have a uniform composition through its thickness which closely resembled that of the starting material. These problems were directly due to the fact that the rates of evaporation of elements contained in the multi-constituent starting material alloy are related to their vapor pressures at the temperature of the evaporation source. In the case of alloys, particularly multi-constituent alloys, one or two elements thereof typically have significantly higher vapor pressures than the others, such that the condensate is richer than the starting material in these elements. If the material being evaporated has a fixed volume and is entirely evaporated, the condensate will have a non-uniform composition throughout its thickness, but will reflect, in a macroscopic sense, the starting composition of the material. If the starting material is continually replenished, such as by maintaining a constant pool volume, the composition of the condensate will be higher throughout its thickness in the elements which have higher vapor pressures.
U.S. Pat. No. 5,474,809 to Skelly et al, assigned to General Electric Company who is the common assignee with respect to the present invention, expressly recognized the problems of the prior art in achieving uniform and desired composition for the coating closely corresponding to that of the evaporated material. Such patent disclosed a method for carrying out vapor deposition that achieved a coating on a base material which closely resembled the composition of the starting (i.e. evaporated) material, that is to say the coating purportedly to contain substantially the same elements in substantially the same proportions as the starting material, even if the starting material was a multi-constituent alloy having elements each of significantly different vapor pressures. In particular, the aforementioned Skelly et al patent disclosed a method of making an evaporated deposit of a material using the vapor deposition process, wherein one material (a second material) having a composition which was desired to be formed as a coating on a base material, is overlaid by a first material which consisted of a refractory material with a higher melting point or a vapor pressure at an elevated temperature that is less than each of the constituents of the second material. Accordingly, upon heating of the first material the underlying second material becomes melted, and the constituent elements of the second material proximate the overlying first material are transported by convection and thermal mixing through the first overlying material and thereafter evaporated from the surface of the first overlying material. In such process the first material becomes molten and transmits heat downwardly to the underlying second material, thereby forming a molten zone therein, and second material in such molten zone therein becomes mixed with the molten zone of first material immediately above it, permitting the second material to be evaporated from the surface thereof. Advantageously, such process purportedly permits coatings to be formed on a base material having a composition which is substantially identical to that of the second material. The second material (or at least certain of the elements therein) which were desired to be evaporated possessed vapor pressures which permitted such elements to be preferentially evaporated in comparison to the first material. Accordingly the deposit contained quantities of the second material, but no or only minute trace amounts of the first material (less than 0.05 atomic percent).
In the case of refractory materials in the form of rare earth materials such as zirconium or hafnium intended to be incorporated into a thermal coating for deposit on a base material, it is actually desirable for such materials to be evaporated and thereby incorporated in the deposited coating where a thermal barrier coating is desired to be applied. However, when rare earth metals, such as zirconium or hafnium are used in the process of Skelly et al described in U.S. Pat. No. 5,474,809 as the first material, and metal alloys such as a nickel-aluminum alloy is used as the second material, it is found that the method taught by Skelly et al is physically unworkable. In this regard, when employing a rare earth metal, such as zirconium, as the first material using the method taught by Skelly et al, such first material when melted tends to xe2x80x9cball-upxe2x80x9d when heated by a heat source such as an electron beam, by virtue of the surface tension forces existing between molten zirconium and the solid second material. As a result there is little or no proper transfer of heat downwardly to the underlying second material to form a molten zone there within so as to permit molten second material to migrate upwardly there through and thereafter evaporate from the surface. In such circumstances, neither the underlying second material or the zirconium which comprises the first material becomes evaporated so as to form a deposit. Moreover, in Skelly et al even where the first material is not a rare earth metal, the Skelly et al patent did not develop or disclose any circumstances in which it was capable of obtaining quantities of the refractory metal in the deposit in concentrations greater than trace amounts (i.e. greater than 0.05 atomic percent).
A workable vapor deposition apparatus and method for incorporating greater than trace amounts of refractory materials such as zirconium and hafnium metals into coatings on base materials for use as thermal barrier coatings and the like is disclosed and claimed.
The present invention in one of its broad embodiments consists of a method of forming a deposit on a base material, such deposit having at least two elements from a second material and small amounts of a refractory element selected from the group of refractory elements comprising zirconium, hafnium, yttrium, titanium, rhenium, silicon, chromium and alloys thereof, comprising the steps of:
selecting a first material comprising said at least two elements further alloyed with said refractory element, said second material comprising said at least two elements, said first material adapted to permit transport of said at least two elements in said second material through said first material when said first and second material are in a molten state and in touching contact with one another so as to permit evaporation of said two elements and said refractory element from an exposed surface thereof;
placing a quantity of said first material over a quantity of said second material in a crucible means so that the first material at least partially covers the second material;
supplying heat to the first material sufficient to create a molten zone within and through the first material such that the molten zone of the first material is in touching contact with the second material to thereby create a molten zone within the second material, wherein said two elements in the second material are transported through the molten zone rich in the first material and said refractory element and said two elements are each evaporated therefrom thereby forming a vapor stream; and
collecting condensate from the vapor stream as a deposit on the base metal.
The method of the present invention, where the first material comprises a refractory material having a high melting temperature such as titanium, zirconium, or hafnium or alloys thereof, has the unexpected and surprising result that, contrary to what would be expected from the teachings of Skelly (if such could be practiced without the xe2x80x9cballing upxe2x80x9d of the rare earth metal, discussed supra) more than trace amounts, namely small amounts and amounts over 0.05 atomic percent of the refractory element may be incorporated into the deposit. In particular, and advantageously, the method of the present invention by providing a refractory element that is alloyed with at least two of the same elements that are intended to be evaporated and form part of the deposit, and further by ensuring that sufficient amounts of refractory element is present in the first material, is able to overcome not only the xe2x80x9cballing upxe2x80x9d difficulties of refractory rare earth metals, but further, in contrast with the result obtained by the method disclosed in U.S. Pat. No. 5,474,809 to Skelly et al, obtain a deposit having greater than simply trace amounts of a refractory material. For the purposes of this document, the definition of trace amounts is the same as adopted in the Skelly et al patent, namely atomic percentages equal to or less than 0.05 atomic percent.
In practicing the method of the present invention, it is necessary that sufficient quantities of the refractory material be present in the first material in order to produce a deposit containing more than trace amounts of refractory material (i.e. more than about 0.05 atomic percent.) It has been found at least from experimental results to date that the atomic percentages of refractory material present in the first material is not particularly directly related to the amount of refractory material present in the deposit, at least for the range of atomic percentages tested, namely the range of 33%-76% (a/o) of refractory element present in such first material, these ranges being the preferred ranges. Rather, the presence of refractory material in atomic percent in the deposited coating appears more related to the physical quantity of refractory material in the first material in proportion to the underlying second material. In other words, there is some indication from the experimental results, while not always holding true, that the greater the physical quantity of refractory material and thus the greater the quantity first material physically covering the second material when enclosed in a crucible, the greater the quantity of the refractory element present in the deposit. However, because this generalization does not appear to always hold true, some experimentation as to the amount of refractory material present in the first material for a given coverage of the second material of a given dimension may be required in order to arrive at a physical amount of refractory material which need be present in the first material in order that the deposit contain greater than trace amounts of such refractory material.
It has been found it is beneficial (although not a requirement) that the first material contain the same two elements in the same relative atomic percentages as in the second material, in order to assist in uniformity of composition of the deposited coating and also to permit the coating composition to more closely match that of the second material. Accordingly, in a first preferred embodiment, the two elements present in the second material (which in the preferred embodiment are nickel and aluminum) are present in approximately equal ratios to each other. Likewise in another preferred embodiment, the two elements present in the first material are also present therein in approximately equal ratios. Preferably, as mentioned above, the two elements present in both the first material and the second material are present in equal ratios in each of the first and second materials, with such ratio being approximately 50xe2x80x9450 (a/o) in the preferred embodiment.
The refractory material is preferably comprised of either zirconium, hafnium, yttrium, rhenium, silicon, chromium, titanium or alloys thereof, although other refractory materials or alloys thereof may be used. Where the deposited coating applied by the above method is intended to be a thermal barrier bond coating, in a preferred embodiment the first material is formed of a rare earth metal such as zirconium or hafnium. Two elements found suitable for these purposes and for alloying thereto in the first material and being present in the second material are nickel and aluminum, and in a further preferred embodiment, the nickel and aluminum exist in alloy in the second material and also in the first material in an approximate molar ratio of 1:1.
Advantageously, using the aforesaid preferred method of the present invention, refractory material may be uniformly deposited on a base material in concentrations exceeding nominal percentages (i.e. exceeding 0.05 atomic percent).
As a further consideration, it has been found that when pure rare earth metals, such as zirconium and hafnium are used as the refractory element, upon being heated have a tendency to form oxide skins. These oxide skins not only have higher melting points than the pure rare earth metals but also further tend to reduce the rate of evaporation of such rare earth metal thus slowing the rate of deposition of any condensate. These oxide skins, due to the higher melting points, greatly reduce the evaporation of rare earth metals, with the result that any deposits formed are virtually devoid of rare earth elements therein.
Accordingly, in a further aspect of the method of the present invention where the refractory element is a rare earth metal, a particular manner of heating is disclosed in order to obtain a deposit with more than simply trace concentration of rare earth metals. In particular, when carrying out the step of supplying heat to said first material, such step comprises supplying heat to an inner heated area and a surrounding outer heated area, wherein at least a portion of said inner heated area is heated to a greater temperature than the surrounding outer heated area. In a preferred embodiment, where an electron beam is used as a means of heating, such heating step comprises directing an electron beam across the first material, further comprising:
directing such a beam across the inner heated area; and
directing such a beam across the outer heated area; and
providing a scanning pattern effective to transiently increase temperature of a least a portion of said inner heated area above that of said outer heated area so as to transiently increase vaporization from said at least a portion of said inner heated area.
The mechanics of how a heating pattern such as that disclosed herein is able to increase the rate of evaporation is not entirely known. It is theorized that by heating the inner heated area to a temperature greater than the surrounding outer area, that convection currents, in particular Maragoni flow patterns, are created in the molten zone, which divert oxide layers which may form at the upper exposed surface to the outer edges of the molten zone surface, thereby leaving unoxidized molten material, including rare earth metal, proximate the inner heated zone surface which can then be better evaporated from the hottest part of the molten material and thereafter condensed on the base material to thereby form the coating.
A base material or article having a coating deposited thereon by the method of the present invention is also disclosed. Advantageously, coatings applied to articles by the method of the present invention possess refractive material therein in excess of nominal percentages.